This invention relates to a method of producing thermistor chips for surface mounting and, in particular, such thermistor chips which may be used for temperature compensation of electronic apparatus or as sensors for measuring surface temperatures.
Thermistor chips have the problem that the exposed areas of the thermistor body become corroded and dissolved when a process of electrolytic plating is carried out on the outer electrodes, thereby causing the value of their resistance to change. Thus, it has been known to form an insulating layer such as a glass layer on the surface of the thermistor body in order to prevent the corrosion of the thermistor body at the time of electrolytic plating. Japanese Patent Publication Tokkai 3-250603, for example, has disclosed a thermistor chip 1 thus produced, as shown in FIG. 7, having outer electrodes 4 formed over two end surfaces of a thermistor element 2 with its outer surfaces completely covered with a glass layer 3 except on these end surfaces.
Such thermistor chips 1 may be produced firstly by printing and baking a glass paste on both main surfaces of a ceramic green sheet to form the glass layers 3 on both main surfaces of a thermistor body 5, as shown in FIG. 8A. After the sintered sheet 6 thus produced is cut into strips 7 by means of a dicing saw, the glass paste is applied by printing and baked also on the cut surfaces to form glass layers 3 thereon, as shown in FIG. 8B. These strips 7 are then cut perpendicularly to these cut surfaces to obtain thermistor elements 2 in chip forms, as shown in FIG. 8C. Baked electrode layers 4a (shown in FIG. 7) are formed by applying and baking an electrically conductive paste on the cut surfaces which are the end surfaces of these thermistor elements 2. Plated layers 4b (shown in FIG. 7) are further formed over these baked electrode layers 4a by an electrolytic plating process to obtain chip thermistors 1 as shown in FIG. 8D wherein numerals 4 indicate electrodes each consisting of a baked electrode layer 4a and a plated layer 4b. 
This method of production is disadvantageous because it includes the step of using a dicing saw to cut the sintered sheet 6 with glass layers 3 formed on both its main surfaces and the step of thereafter applying and baking a glass paste on the exposed cut surfaces and hence is complicated and costly.
It is therefore an object of this invention to provide a method of producing thermistor chips with a new structure with insulated surfaces.
Thermistor chips having outer electrodes on mutually opposite end surfaces of a thermistor element and diffused layers of an inorganic material having a higher specific resistance than the material of the thermistor element formed proximally to externally exposed areas of the thermistor element may be produced by preparing ceramic green sheets, applying an inorganic material such as a glass paste on them in areas including lines along which they are later to be cut, stacking a plurality of these green sheets one on top of another to obtain a stacked body, obtaining chips by cutting this stacked body along the aforementioned lines and subjecting these chips to a firing process to obtain a sintered body, and forming outer electrodes on mutually opposite end surfaces of the sintered body.